Not being a lot of help here, but I measured / calculated the input impedance of the MDA300 ADC input recently, and it gave me 500kOhms.
> -----Original Message----- > From: [email protected] > [mailto:[email protected]]on Behalf > Of Michael > Schippling > Sent: 08 November 2010 16:31 > To: krisa > Cc: [email protected] > Subject: Re: [Tinyos-help] ADC reading fluctuations > > > You may be getting power line noise. Try shorting the ADC input > to ground to see if the fluctuation goes away. If not there may > be a problem with your power supply itself -- although the > accelerometer reading would indicate otherwise. If the noise > goes away when shorted you can try shielding the sensor wires > and fiddling with the wiring and routing. It could also be a > ground loop if you aren't connecting your ground wires to a > common point. > > I'm not clear how you are connecting your bend sensor. If it > is a "straight-ahead" variable resistance, you need to have > a fixed resistor in series.... Oh, maybe that's the issue.... > Are you just running it between the ADC input and ground? > If so a better way would be to have a resistor from the > Vref or power supply to the sensor, who's other end is > grounded. Then attach the ADC input to the resistor-sensor > connection point. That makes the whole thing a voltage divider. > Start with a 10K resistor, since that's how much your bender > changes. You can use ohms law to figure out a more optimal > value based on the min/max resistance of the sensor. Bear in > mind that the ADC input has an impedance which will be in > parallel with your sensor, IIRC, something around 250Kohms. > > MS > > krisa wrote: > > I am having a problem with reading value from ADC ports of > telosb mote > > using TinyOS 2.1.1 > > I have a simple variable resistor sensors (bend sensors) attached to > > ports 6.3 - 6.7 (ADC3-ADC7), and while their resistace is > not changed > > I would assume their readings to be somewhat constant (of > course with > > some noise), but they are fluctuating in almost a sine like > pattern - > > while sampling with 100ms rate some 3-4 readings have a value which > > seems lke the correct value, then some 4-5 readings have value lower > > by some 200-300 or even more. > > On the other hand I have an accelerometer attached to ADC0-ADC2 and > > its readings while in peaceful condition only fluctuates a little > > (maybe 10-20 units max) and not in such a recognizable > pattern - which > > seems as it should be. > > > > I am using this configuration for reading the values > > const msp430adc12_channel_config_t config = { > > inch: INPUT_CHANNEL_A0, //Of course changing to the > correct port > > sref: REFERENCE_VREFplus_AVss, > > ref2_5v: REFVOLT_LEVEL_2_5, > > adc12ssel: SHT_SOURCE_ACLK, > > adc12div: SHT_CLOCK_DIV_1, > > sht: SAMPLE_HOLD_4_CYCLES, > > sampcon_ssel: SAMPCON_SOURCE_SMCLK, > > sampcon_id: SAMPCON_CLOCK_DIV_1 > > }; > > but I have experimented with many others - by some manipulations to > > clock selection or sample hold time, I have managed to get constant > > readings, but then the readings dont change even if the > resistor value > > is changed - basically not working. > > I have tried both reading thorough the Read<uint16_t> interface and > > Msp430Adc12MultiChannel through Msp430Adc12ClientAutoRVGC with the > > same results - maybe there is another method? > > > > Also - even if I disconnect the sensor from the port i still get > > uneaven readings - sometimes maximum of 4095 and sometimes as low as > > 0. > > I am quite sure that the problem is me not knowing how to read the > > sensor data in TinyOS and not broken mote, because It was previously > > tested with another programming language which gave more consistent > > sensor readings. > > The only possiblity what comes to my mind is that there is some kind > > of inconsistency with clocks (some interferance pattern?) > because the > > readings are distorted in a regular pattern and if I change the > > sampling rate the distortions also still are sine form only > longer or > > shorter. (1000 ms sampling yields some 9 high and 9 low readings, 25 > > ms sampling yields approx 2 high and 2 low readings). > > > > I think the problem is the configuration, and seeing as I > am quite new > > to mote programming I am a little in dark here - which reference and > > clock should I use (it seems they could be different from those used > > for the accelerometer) > > > > Also a followup question - the resistor when changed has > approx 10kohm > > variance which translates in approx 100-200 points of > reading change - > > is there a way to enhace this so the range would be closer to the > > whole scale of 0-4095? > > > > I have spent many days on this problem and would really > appreciate any > > help or insight in this matter. > > > > Thank you in advance! > > > > > _______________________________________________ > Tinyos-help mailing list > [email protected] > https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/t inyos-help Please consider the environment before printing a hard copy of this e-mail. 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